Electro-mechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing and printing. Optical beam processing for printing with deformable mirrors has been described by L. J. Hornbeck, see U.S. Pat. No. 4,596,992, "Linear spatial light modulator and printer," issued on Jun. 24, 1984. A device for optical beam modulation using cantilever mechanical beams has also been disclosed, see U.S. Pat. No. 4,492,435, "Multiple array full width electro-mechanical modulator," issued on Jan. 8, 1985 to M. E. Banton and U.S. Pat. No. 5,661,593, "Linear electrostatic modulator," issued on Aug. 26, 1997 to C. D. Engle. Other applications of electro-mechanical gratings include wavelength division multiplexing and spectrometers, see U.S. Pat. No. 5,757,536, "Electrically programmable diffraction grating," issued on May 26, 1998 to A. J. Ricco et al. Electro-mechanical gratings are well known in the patent literature, see U.S. Pat. No. 4,011,009, "Reflection diffraction grating having a controllable blaze angle," issued on Mar. 8, 1977 to W. L. Lama et al and U.S. Pat. No. 5,115,344, "Tunable diffraction grating," issued on May 19, 1992 to J. E. Jaskie. More recently, Bloom et al described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV), see U.S. Pat. No. 5,311,360, "Method and apparatus for modulating a light beam," issued on May 10, 1994. This device was later described by Bloom et al with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate, 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast, 3) solid supports to fix alternate ribbons, and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces, see U.S. Pat. No. 5,459,610, "Deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate," issued on Oct. 17, 1995, and U.S. Pat. No. 5,808,797, "Method and apparatus for modulating a light beam," issued on Sep. 15, 1998. Bloom et al also presented a method for fabricating the device, see U.S. Pat. No. 5,677,783, "Method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate," issued on Oct. 14, 1997.
Another disclosure in Bloom et al '610 was the use of a patterned ground plane in order to realize two-dimensional arrays. Two embodiments were disclosed: the use of a refractory metal on an insulated substrate and selective doping of a semiconducting substrate to create a p-n junction. The purpose of that invention was to create an array of ground electrodes corresponding to the array of grating elements to enable two-dimensional addressing, as opposed to allowing two different voltage levels to be applied below the ribbon elements. J. G. Bornstein et al also disclosed the use of a patterned ground plane, using a patterned refractory metal on an insulator, in order to address a two-dimensional grating element array in U.S. Pat. No. 5,661,592 entitled "Method of making and an apparatus for a flat diffraction grating light valve," issued on Aug. 26, 1997.
According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. By modulating the voltage waveform, it is possible to modulate the diffracted optical beam as needed by the specific application. However, a single polarity voltage waveform can lead to device operation difficulties if leakage or injection of charge occurs into the intermediate dielectric layers between the ground plane and the conductor on the ribbons.
One method to alleviate this problem is to provide an alternating voltage to the ribbons. A DC-free waveform produces nearly the same temporal modulation of the diffracted optical beam as the corresponding single polarity waveform while minimizing charge accumulation in the dielectric layers. Stable device operation is thus achieved. However, this complicates the driving circuitry requiring bipolar rather than unipolar driving capability.